Circuit for comparing high frequency amplitude modulated signals

ABSTRACT

A comparison circuit associated with a video tape recorder having two identical legs (41, 42), one of which (41) is connected to a head of a first type and the other (42) to a head of a second type, the outputs of both being connected to a respective input of a comparison means (56) for selecting in &#34;search&#34; mode the head which presents the highest signal amplitude. Each branch comprises, in series, a high-pass filter (45), a variable gain amplifier (46), a multiplier (47), a low-pass filter (48) and a capacitance amplifier (49).

BACKGROUND OF THE INVENTION

The present invention relates to circuits for comparing amplitudemodulated signals and more particularly circuits for comparing signalsfrom video tape recorder heads.

Some video tape recorders (VTR) comprise four heads, two for a "normalduration" mode referenced S1 and S2 which serve both for recording andreading, and two for a "long duration" mode referenced L1 and L2 whichalso serve both for recording and reading. Generally, the duration of arecording or reading operation in the latter mode is twice the "normalduration" mode.

The four heads are integral with a circular drum 1 schematically shownin the top view of FIG. 1. The two heads S1 and S2 are diametrical aswell as heads L1 and L2. Moreover, heads L1 and L2 are shifted by anangle α generally low, of about one thousandth radian, with respect toheads S2 and S1, respectively.

The VTR heads record or read data (picture and sound) on a magnetic tape2 shown on FIGS. 1 and 2. Data are contained in rectilinear tracks 3which exhibit a small angle β with respect to the longitudinal axis ofthe tape. Tracks 3 are arranged side by side.

In operation, the drum rotates around its axis and the tape advanceswhile being pressed against the drum over more than 180° (FIG. 1). Theheads are actuated at a relative speed with respect to the tape. In the"normal duration" mode, each of heads S1 and S2, one after the other,interacts with a single track. In the "long duration" mode, each ofheads L1 and L2 interacts with a single track one after the other.

On the tape portions not pressed against the drum, referenced B1 and B2in FIG. 1, the longitudinal axis of the tape is perpendicular to thedrum axis. On portion B3 of the tape pressed against the drum, thelongitudinal axis of the tape at each point presents with the drum axisan angle slightly different from π/2 so that, for a relative head speedadapted with respect to the tape, the heads follow a path T1, shown inFIG. 2, parallel to the longitudinal axis of the tracks.

In addition, such four-heads VTRs can generally operate in an additionalmode, called "search" mode, wherein pictures are presented at anaccelerated speed for rapidly looking for specific sequences. In such amode, the tape advance speed is increased and the path of the heads onthe tape is no longer parallel to the longitudinal axis of the tracks.Such a path T2 of a "search" mode head is shown in dotted lines in FIG.2. In this event, a head advances, on the one hand, inside tracks and,on the other hand, on areas separating tracks. The passage of the headson the separation areas impairs the picture quality. Indeed, in "normalduration" mode or in "long duration" mode, the signal obtained from thetracks during reading is a high frequency signal (100 kHz to 10 MHz)with a constant amplitude. In "search" mode, the amplitude of the highfrequency signal will decrease each time the head cuts a separation areabetween tracks. Thus, the resulting signal will be a high frequencyamplitude modulated signal. The amplitude decreases at the areasseparating tracks correspond to noise bars on pictures.

Then, it has been devised, in the prior art, for eliminating those noisebars, a method wherein one no longer uses, in "search" mode, only theS-type heads ("normal duration" mode) or only the L-type heads ("longduration" mode). Since the picture degradation corresponds to signaldecreases, a circuit has been provided for comparing the amplitude ofthe signals associated with the head S with the amplitude of the signalsassociated with the head L of a couple of adjacent heads on the drum(couples (S1, L2) or (S2, L1)) and for selecting the head for which theamplitude is the highest. Moreover, in video tape recorders implementingsuch an approach, the signals from heads S and heads L are phase shiftedby a value very close to π. This can be obtained by adjusting theadvance speed of the tape. Performances are thus optimized, a decreasingsignal being replaced by an increasing signal, and so on.

Such a circuit is shown in FIG. 3. It comprises two identical legs 21and 22, leg 21 receiving at its input 23 signals from a "normalduration" head S and leg 22 receiving at its input 24 signals from a"long duration" head L.

An amplifier 25 is provided at the input of each leg. The output of thisamplifier is connected to the input of a low-pass filter 26 used foreliminating noise peaks. A detector 27, usually formed by a diode D, acapacitor C1 and a resistor R, has its input connected to the output ofthe low-pass filter. This detector has a time constant R×C1 and suppliesa signal corresponding to the upper envelope of the high frequencymodulated signal. The output of the detector constitutes the leg output.

The output of each leg is connected to an input of a same comparator 28.According to the sign of the voltage difference between its two inputs,the comparator supplies a logic "1" (generally the supply volt 5 V) or alogic "0" (generally 0 V), which determines the selection of the headassociated with the maximal amplitude.

However, the circuit of FIG. 3 presents a limited sensitivity. Thevoltages of the input signals are liable to reach 600 mV undersatisfactory conditions, and they may not exceed 60 mV in case of poorquality recording, for example. If it is desired to significantlyamplify the low intensity signals, by choosing an amplifier gain equalto 15 for example, the signals at the amplifier output will reach 5 V incase of signals initially having a high intensity. This would cause theamplifier to operate in saturation mode. Thus, the gain must not be toohigh, and not exceed 7 or 8.

In addition, the detector must have a high time constant R×C1 to ensureproper processing, usually about 1 ms. The resistance is about 10 kΩ andcapacitance C1 is about 100 nF. Resistor R is integrable but capacitorC1 is too large to be integrated; it should be lower than about 10-20pF. For decreasing C1 to such a value, the order of magnitude of Rshould be 100 MΩ, which is very high and would produce a very scatteredintegrated resistor. The circuit of FIG. 3 is therefore not completelyintegrable.

An object of the invention is to provide a circuit for comparing highfrequency amplitude modulated signals with a high sensitivity.

Another object of the invention is to provide such a fully integrablecomparison circuit.

SUMMARY OF THE INVENTION

To achieve those objects, the invention provides a circuit for comparingsignals of a video tape recorder capable of operating in a first "normalduration" mode by using a first type of heads, in a second "longduration" mode by using a second type of heads and in a third "search"mode. This comparison circuit has two identical legs for processingsignals; the input of one of the legs is connected to a head of thefirst type and the other to a head of the second type, and the output ofeach leg is connected to an input of a same comparator for selecting in"search" mode that of the two heads which has the highest amplitude.Each leg comprises, in series, a high-pass filter, a variable gainamplifier, a multi-plier multiplying a signal by itself, a low-passfilter and a capacitance amplifier comprising a second comparatorsupplying a current at the output, the first input of which constitutesthe input of the capacitance amplifier and the second input isconnected, on the one hand, to the ground through a capacitor and, onthe other hand, to the output of the capacitance amplifier. The input ofthe high-pass filter constitutes the leg input and the output of thecapacitance amplifier constitutes the output of the leg which isconnected to the variable gain amplifier to control the gain thereof.

BRIEF DISCLOSURE OF THE DRAWINGS

The foregoing and other objects, features and advantages of theinvention will be apparent from the following detailed description of apreferred embodiment as illustrated in the accompanying drawingswherein:

FIG. 1, already described, is a very schematic top view of a four-headvideo tape recorder drum;

FIG. 2, already described, shows a tape used in a video tape recorder;

FIG. 3, already described, shows a comparison circuit according to theprior art;

FIG. 4 shows a comparison circuit according to the invention;

FIG. 5 shows signals at different steps of processing by the comparisoncircuit of FIG. 4; and

FIG. 6 shows a variant of the comparison circuit of FIG. 4.

DESCRIPTION OF A PREFERRED EMBODIMENT

FIG. 4 shows a circuit for comparing high frequency amplitude modulatedsignals according to the invention. It comprises two identical legs 41and 42. Leg 41 receives at an input 43 signals from a head S ("normalduration" mode) and leg 42 receives signals from a head L ("longduration" mode) on an input 44.

The input of each leg is constituted by the input of a high-pass filter45, the output of which is connected to the input of a variable gainamplifier 46. The output of amplifier 46 is connected to the input of amultiplier 47. A low-pass filter 48 has its input connected to theoutput of the multiplier and its output connected to the input of acapacitance amplifier 49. The latter conventionally comprises acomparator 50 supplying an output current, a first terminal 51 of which,or positive input, constitutes the input of the capacitance amplifierand the second input 52, or negative input, is connected, on the onehand, to the terminal of a capacitor C2, the other terminal of which isgrounded, on the other hand, to the output 53 of comparator 50. Output53 is feedback connected to a gain control input 54 of amplifier 46. Inaddition, the output of the capacitance amplifier constitutes the legoutput.

The outputs of legs 41 and 42 are connected to the + and - inputs of acomparator 56, the output 57 of which forms the output of the comparisoncircuit.

The voltage signal from the head S received at the input terminal 43 ofthe circuit is represented as a function of time on curve G1 of FIG. 5.The signal from the head L applied to the input terminal 44 of thecircuit is represented on curve G2 of FIG. 5. The input signals are highfrequency amplitude modulated signals of sinusoidal type. Their phaseshift is represented equal to π.

The high-pass filter 45 has for example a cutoff frequency of 1 MHz. Thesignal going out of the filter is amplified by the variable gainamplifier 46; the control of this gain is explained hereunder. Then, inmultiplier 47, the signal is multiplied by itself. Since the incomingsignal is sinusoidal, the signal supplied by the multiplier comprises ad.c. component and even harmonics of the input frequency. The low-passfilter 48, with a cutoff frequency of 100 kHz, transmits only this d.c.component.

Without the presence of the high-pass filter at the input of the leg,the double frequency harmonics would have a minimum frequency of 200 kHzsince the signals from the heads vary from 100 kHz. By using thehigh-pass filter 45, this minimum frequency is 2 MHz. Filtering of thelow-pass filter is thus more efficient by providing the high-passfilter.

The d.c. component at the output of the low-pass filter varies as afunction of time since the signal from the heads is amplitude modulated.The frequency of this signal is about 10 KHz. The latter is introducedinto the capacitance amplifier 49 which is a device conventionally usedfor smoothing an incoming signal. When the voltage difference betweeninputs 51 and 52 of comparator 50 is positive, the latter supplies atits output a current I. As, due to their design, comparators 50 and 56have very low input currents with respect to their output currents,current I only serves to charge capacitor C2. The capacitor charge isrealized according to the relation:

    I=C2(ΔV/ΔT)

where ΔV is the voltage difference across the terminals of the capacitorand ΔT the charge duration. For a voltage difference ΔV, one determinesa time constant given by

    τ=(C2/I)ΔV.

A satisfactory smoothing can be obtained for a high time constant, ofabout 10 μs. With adapted values ΔV=1V and I=1 μA, capacitor C2 has avalue C2=10 pF. Such a capacitor is integrable.

The smoothed signal present at the output of the capacitance amplifieris fed back to amplifier 46 in order to control the gain thereof. Thisamplifier has a gain which varies substantially linearly between a highvalue when the feedback control signal is low and a low value when thecontrol signal is high.

Because of the gain control of amplifier 46, and, since signalamplitudes raised to the second power are considered, the signal at theoutput of a leg will have the shape illustrated on curves G3 (leg 41)and G4 (leg 42) of FIG. 5. Signals associated with curves G3 and G4 areintroduced into comparator 56.

The gain of amplifier 46 may range from 1 to 20 for example. Whenamplitude (a) of the signal at the output of amplifier 46 is low, thegain is high (close to 20) but the component raised to the second ordera² remains relatively low, as shown in a portion P of curve G4. On aportion Q, amplitude a is higher and the gain has decreased to a valuewhich is liable to vary around 10. Due to the fact the amplitude issquared, an increase in signal is obtained, as well as an increase inthe variation speed of this signal. Thus, since the selection of eitherleg (crossing of curves) is mainly realized near such portions Q of thecurves since the shift drift between the latter is about π, lowestintensity signals will be compared and sensitivity is increased. Inaddition, since the variation speed is higher, the selection is madewith a better accuracy.

Lastly, when the amplitude a is close to its maximum, the gain is low;it is close to 1. Curves exhibit a flattened shape (portion R of curveG4). Signals from the heads with an amplitude of 0.6 V are compatiblewith the comparison circuit, this high value raised to the second powerremaining lower than the admitted limit values.

Thus, the circuit according to the invention, in addition to itsincreased sensitivity, accepts high intensity signals. In addition,since capacitor C2 can be integrated, the circuit is completelyintegrable.

Comparator 56 shown in FIG. 4 supplies at its output a high level signal(5 V) if the intensity of the signal from leg 41 is higher than that ofthe signal from leg 42, and it supplies a low level signal (0 V) in theinverse case. This permits to determine the selection of the headassociated with the maximum amplitude. The output signal of thecomparator is represented by curve G5 in FIG. 5.

In the circuit of FIG. 3, the time constant of detector 27 is about 1ms. It is not possible to replace this detector by a capacitanceamplifier in order to form an integrated circuit since the capacitorwould have a too high value (for a current of 1 μA, it would benecessary to have a capacitance of 100 nF).

The S-type heads have different features than those of L-type heads. Thesignal to noise ratio associated with heads S is higher than thatassociated with heads L. For low intensities liable to be detected bythe circuit according to the invention because of its high sensitivity,it could be desirable to select head S rather than head L while thelatter supplies signals having a higher amplitude.

This is achieved, according to a variant of the invention, by replacingcomparator 56 of FIG. 4 by a device 71 shown in FIG. 6. Device 71comprises a comparator 72 which receives on a first input 73, ornegative input, the signal from leg 41 and on a second input 74, orpositive input, the signal from leg 42. In device 71, the positive andnegative inputs of comparator 72 are inverse with respect to those ofcomparator 56 of FIG. 4.

Each of inputs 73 and 74 is connected to a positive input of aconventional three-inputs hysteresis comparator 75, a negative input ofwhich is connected to a voltage source 76 which supplies a referencevoltage Vr. Each of outputs 77, 78 of comparators 72 and 75,respectively, is connected to an input of an NAND gate 79, the output 80of which constitutes the output of device 71.

If the voltages on the two inputs 73 and 74 are higher than thereference voltage Vr, or if the voltage on one only of those two inputsis higher than Vr, the output of the hysteresis comparator is set to 1.Then, there is again on the output of device 71 a high signal when thesignal associated to head S is higher than the signal associated withhead L, and inversely. In that case, the circuit operation of FIG. 4 isfound again.

On the contrary, if the voltages on the two inputs 73 and 74 are lowerthan the reference voltage, the output 78 of the hysteresis comparatoris set to 0. Then, the output 80 of the NAND gate is set to 1. Thus, thehead S is selected when the two signals at the output of legs 41 and 42are lower than a reference voltage, even if the signal associated withhead L is higher than the signal associated with head S.

We claim:
 1. A comparison circuit associated with a video tape recorderable to operate in a first "normal duration" mode by using a first typeof heads, in a second "long duration" mode by using a second type ofheads and in a third "search" mode, this comparison circuit having twoidentical legs, an input of one leg being connected to a head of thefirst type and an input of the other leg to a head of the second type,outputs of both legs being connected to a respective input of acomparison means for selecting in "search" mode the head which receivesa highest signal amplitude, wherein each leg comprises, in series, ahigh-pass filter, a variable gain amplifier, a multiplier multiplying asignal applied thereto by itself, a low-pass filter and a capacitanceamplifier for smoothing an output signal of said low-pass filtercomprising a first comparator supplying an output current of saidcapacitance amplifier, a first input of said first comparatorconstituting an input of the capacitance amplifier and a second inputbeing connected to a ground through an integrated-circuit capacitor andto an output of said capacitance amplifier, an input of the high-passfilter constituting the leg input and the output of the capacitanceamplifier constituting the leg output which is connected to saidvariable gain amplifier for gain control.
 2. A comparison circuitaccording to claim 1, wherein the gain of said variable gain amplifierof each leg ranges from 1 to
 20. 3. A comparison circuit according toclaim 1 wherein said comparison means which comprises a secondcomparator having a negative input and a positive input each of which isconnected to a positive input of a third three-input hysteresiscomparator, the third input of which is connected to a source of areference voltage, each of the outputs of the second and thirdcomparators, respectively, being connected to an input of an NAND gate,the output of which constitutes the output of said comparison means,whereby the signal associated with the negative input of the secondcomparator is selected when the voltages on the two inputs of thissecond comparator are lower than said reference voltage.
 4. A comparisoncircuit associated with a video tape recorder able to operate in a first"normal duration" mode by using a first type of heads, in a second "longduration" mode by using a second type of heads and in a third "search"mode, this comparison circuit having two identical legs, an input of oneleg being connected to a head of the first type and an input of theother leg to a head of the second type, outputs of both legs beingconnected to a respective input of a comparison means for selecting in"search" mode the head which receives a highest signal amplitude,wherein each leg comprises, in series, a high-pass filter, a variablegain amplifier, a multiplier multiplying a signal applied thereto byitself, a low-pass filter and a capacitance amplifier comprising a firstcomparator supplying an output current of said capacitance amplifier, afirst input of said first comparator constituting an input of thecapacitance amplifier and a second input being connected to a groundthrough a capacitor and to an output of said capacitance amplifier, aninput of the high-pass filter constituting the leg input and the outputof the capacitance amplifier constituting the leg output which isconnected to said variable gain amplifier for gain control, and whereinsaid comparison means comprises a second comparator having a negativeinput and a positive input each of which is connected to a positiveinput of a third three-input hysteresis comparator, a third input ofwhich is connected to a source of a reference voltage, each of theoutputs of the second and third comparators, respectively, beingconnected to an input of an NAND gate, an output of said NAND gateconstitutes an output of said comparison means, whereby a signalassociated with the negative input of the second comparator is selectedwhen voltages on the two inputs of this second comparator are lower thansaid reference voltage.
 5. A comparison circuit according to claim 4,wherein the gain of said variable gain amplifier of each leg ranges from1 to
 20. 6. An integrated-circuit apparatus for processing signals fromvideo tape recorder heads of first and second types, respectively, saidapparatus, comprising:first and second circuit legs corresponding,respectively, to said recorder heads of first and second types, eachincluding (a) high-pass filtering means for high pass filtering signalsfrom its respective head of first or second type, (b) means responsiveto the filtered signals for extracting a direct current component of thefiltered signals, (c) smoothing means for smoothing output signals ofsaid extracting means, and (d) means responsive to output signals ofsaid smoothing means for controlling a gain of the leg; and comparisonmeans for receiving output signals from said two legs, and in response,supplying an output signal, said comparison means including: a signalcomparator for comparing the output signals of the legs; a hysteresiscomparator, having inputs connected to inputs of said signal comparator;a source of a reference voltage connected to another input of saidhysteresis comparator; and means for logically combining output signalsof said signal comparator and said hysteresis comparator.
 7. Anapparatus according to claim 6, wherein an input of said signalcomparator is selected when the voltages on the inputs of said signalcomparator are lower than said reference voltage.
 8. An apparatusaccording to claim 6, wherein said extracting means comprisesmeans forsquaring signals, and low-pass filtering means for receiving an outputof said squaring means.
 9. An apparatus according to claim 6, whereinsaid smoothing means comprises a capacitance amplifier.
 10. An apparatusaccording to claim 9, wherein said capacitance amplifier comprises anintegrated-circuit capacitor.
 11. An apparatus according to claim 6,wherein said gain controlling means includes a variable gain amplifier.12. An apparatus according to claim 11, wherein the gain of saidvariable gain amplifier of each leg ranges from 1 to 20.